1. Field of the Invention
The present invention relates to an electroabsorptive modulator (EAM) more particularly to an EAM in which the optical mode is optimized to minimize the optical insertion losses.
2. Description of the Prior Art
Electroabsorptive modulators (EAM) are known to be used for modulating RF signals. There are many known benefits associated with optical modulation of RF signals including higher frequencies, immunity to electromagnetic interference, and relatively wide band width.
Such EAMs are relatively well known in the art. Examples of such EAM are disclosed in U.S. Pat. Nos. 4,525,687; 4,847,573; 5,107,307; 5,113,283; 5,165,105; 5,402,259; and 5,522,005, all hereby incorporated by reference. Such EAMs are also discussed in the literature--"Theoretical Design Optimization of Multiple Quantum Well Electroabsorption Waveguide Modulators", by N. K. Chin and W. S. C. Chang, IEE Journal of Quantum Electronics, Vol. 29, No. 9, Sep. 19, 1993, pgs. 2476-2488.
Such EAMs are typically formed as a semiconductor waveguide on a substrate. For example, as disclosed in U.S. Pat. No. 5,402,259, assigned to the same assignee as the assignee of the present invention, an electroabsorptive modulator is disclosed which includes a semiconductor waveguide formed on a GaAs substrate. The waveguide consists of one or more GaAs quantum wells sandwiched between two AlGaAs waveguide layers. Ohmic contacts are formed on the device to enable the RF signal as well as any DC bias signal to be connected to the device. As discussed in U.S. Pat. No. 5,402,259, the DC bias signal is used to cause the EAM 24 to operate in its linear range. As is known in the art, the electric field causes a change in the optical absorption of the device, which, in turn causes the intensity of the light to be modulated. By applying an RF signal to the device, the intensity modulation of the input light signal will vary in accordance with the variation of the RF signals.
An optical modulation system incorporating an EAM is illustrated in FIG. 1 and generally identified with the reference numeral 20. An optical carrier, for example, from a laser transmitter 22, is directed to input port of the EAM 24. An optical output port of the EAM 24 is directed to a photodiode receiver 26 by way of an optical fiber 28. An RF input signal and a DC bias are applied to the electrical inputs of the EAM. As discussed above, the EAM 24 modulates the optical carrier as a function of the RF input signal. The photodiode receiver 26 demodulates the optical signal to provide an RF output signal. A DC bias signal is used to bias the photodiode 26.
The performance of the optical modulation system 20 is dependent upon the optical power of the modulated light signal directed to the photodiode receiver 26, relative to the optical power of the optical carrier, for example from the laser transmitter 22. The RF gain of the system increases as a function of the square of optical power directed to the photodiode receiver 26. Therefore, in order to improve the performance of the optical modulation system 20, it is necessary to minimize optical insertion losses in the system. In particular, it is necessary to minimize optical insertion losses between the laser transmitter 22 and the output optical fiber 28. Unfortunately in many known EAMs optical mode mismatches exist between the optical fiber 28 and the EAM 24 which result in significant optical insertion losses which decrease the performance of the overall optical modulation system 20. More particularly, known single mode semiconductor waveguides have mode field dimensions which are several microns in the lateral direction (parallel to the epitaxial layers) and significantly smaller dimensions in the transverse direction (perpendicular to the epitaxial layers). However, conventional single mode optical fibers have mode field diameters closer to 10 microns. Such a mismatch in the mode field dimensions results in relatively significant optical insertion losses which degrades the overall performance of the optical modulation system 20.